There has been known a current supplying device for the rotor winding of an electric machine (cf., e.g., the Inventor's Certificate No. 256,043), comprising a slip ring made of high strength steel and intended for mounting onto the rotor shaft of an electric machine with an electrically insulating layer therebetween. This device also comprises a distribution ring which is mounted on the end of the slip ring and to which a current supplying bus bar electrically connected with the rotor winding of an electric machine is secured. The distribution ring is made of electrically highly conductive material and electrically contacts the slip ring through an additional electrically conductive layer formed as a number of segment shaped spacers which are mounted in the abutting arrangement around the circumference between the ends of the slip ring and distribution ring, the electric conductivity of each spacer increasing as it recedes along an arc from the point of attachment of the current supplying bus bar to the distribution ring to provide a uniform current distribution between the slip ring and the distribution ring. The brushes of the current collecting bus bars of the yoke of the electric machine brush contact device contact the outer periphery surface of the slip ring.
However, the known construction of the current supplying device for the rotor winding of the electric machine has a number of disadvantages.
Firstly, although the electric current flowing between the current collecting bus bar of the yoke of the brush contact device and the current supplying bus bar is rather uniformly distributed around the circumference of the end surface of the slip ring, it flows from the periphery surface of the slip ring, facing the brushes, to the end surface of the same ring, facing the distribution ring, along the paths of different lengths. This results in a nonuniform distribution of current across the slip ring width and over the brushes, if the latter are arranged in a number of rows, especially as the current flows through the material of that ring, which is steel of a high electric resistivity. This, in turn, results in nonuniform mechanical and electrical wear of brushes located in different rows and a subsequent nonuniform heating thereof as well as in nonuniform wear and consequent heating of the slip ring itself. As a result, equal operating conditions of all the brushes and of all the portions of the slip ring periphery surface are not provided, which decreases the reliability of operation of the brush contact device of the electric machine.
Secondly, among the disadvantages of the known construction is the fact that the segment shaped spacers are formed of a meterial with an increased specific electric resistance compared to the material of the distribution ring, and that contact electric resistances exist at the interface of the spacers with high exciting currents of the electric machine, this results in some increase in heat losses of the power supplied to the exciting winding.
Thirdly, the known construction of the current supplying device involves certain difficulties in manufacturing of the constituent elements and subsequent assembling thereof, since to provide a satisfactory operation of the construction it is necessary to utilize a definite number of the spacers having different conductivities and with the position of each spacer required to be carefully fixed by means of special mounting members. In order to obtain a value of the contact resistance of these spacers, not exceeding the predetermined one, and to decrease this value, which is necessary to minimize the spacer heating during operation of the electric machine, it is desirable to impose stringent processing requirements on the finish and precision of machining of the contacting surfaces of the spacers as they are manufactured, and on the careful matching of the spacers to one another when assembling the device.
And lastly, if said rings of this construction are provided with longitudinal ventilating channels, the current carrying capacity of the spacers and, consequently, the efficiency of their use decrease, since in this case, at the points of location of the openings the useful section of the end surfaces of the slip ring, the spacers, and the distribution ring is lost.